1. Field of the Invention
The present invention relates generally to a gate turn-off thyristor suitable for stably turning off a large anode current at high speed and more specifically to a thyristor turning-off gate controlling circuit connected to the gate turn-off thyristor.
2. Description of the Prior Art
As is well-known, gate turn-off thyristors (referred to as GTO thyristors hereinafter) have widely been used in various power electronic fields in place of ordinary thyristors. This is because the GTO thyristors can be turned off without use of any circuits including at least one conductor. Additionally, the GTO thyristor is high in efficiency and small in size, and further can control a high voltage and a large current as an excellent switching element. By the way, one of the important characteristics required for a GTO thyristor is the magnitude of controllable anode current which can be turned on or off in response to gate signals. In particular, once the GTO thyristor fails in the turning-off operation, since there always exists a danger that the GTO is destroyed, it is desirable that the controllable anode current is as large as possible.
However, the magnitude of the controllable anode current is affected by the internal and external conditions of the GTO thyristor. For instance, in the case where a Snubber circuit (surge voltage absorption circuit) connected in parallel with the GTO thyristor includes a small capacitance, the anode voltage rises sharply at high rising rate (dV.sub.A /dt) when the GTO is being turned off, thereby resulting in turn-off failure or GTO thyristor destruction. On the other hand, in the case where the Snubber circuit includes a large stray inductance between wires, a high surge voltage is inevitably superimposed upon the anode voltage, thereby resulting in turn-off failure or GTO thyristor destruction. Therefore, there exists another problem such that the capacitance and the inductance of the Snubber circuit should be restricted, whenever a large anode current is turned off.
A more detailed description of the prior-art GTO thyristor will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.